An earthmoving vehicle powertrain is modeled as a MIMO hydrostatic system. Its control inputs include the engine throttle, the electro-hydraulic pump displacement and the flow valve commands. These inputs offer a great control freedom but need to be coordinated. A coordinated MIMO controller for a multi-load powertrain is designed by using an LQG/LTR technique. Modeling and control design are accomplished and validated by both first principles and experimental data. The experimental data are obtained upon an Earthmoving Vehicle Power-train Simulator (EVPS) at the University of Illinois. The paper details the utilization of this 'Hardware-In-the-Loop' testbed in investigating generic power-optimization problems for hydrostatic transmissions. Experimental results are presented.